1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus, and particularly, to a sheet feeding device that lifts an end portion of an uppermost sheet among a plurality of sheets loaded in a loading tray using a blowing unit and transports the sheet to an image forming unit, and an image forming apparatus with the sheet feeding device.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2007-045630 discloses a sheet feeding device that rapidly feeds sheets one by one to an image forming unit of an image forming apparatus. The sheet feeding device separates the uppermost sheet of a sheet bundle using air from a blowing unit and transports the sheet. The basic configuration of the sheet feeding device is as follows. FIG. 7 is a cross-sectional view illustrating a configuration of a general sheet feeding device. In a sheet feeding device 20, a wind is blown from a blowing unit 1 such as a blower toward the front end portions and the side end portions of the plurality of sheets loaded in a sheet feeding tray 21, air is sent between the sheets by the blowing wind so that the sheet is lifted up to the height of a belt 2a of a suction belt unit 2, and then the uppermost sheet is sucked to and transported by the belt. Subsequently, the sheet is transported to the image forming unit by the suction belt unit 2, and an image is formed on the sheet.
Here, the blowing unit 1 serves to not only lift the sheet up to a predetermined height by sending air between the sheets, but also separate the sheets by blowing a wind toward an area extending in the vertical direction by a predetermined amount. A sheet blocking member 7 is disposed between the blowing unit 1 and the sheets, and prevents the sheets other than the uppermost sheet P1 from being transported.
Further, in order to constantly maintain a distance h between the belt 2a, and the position of the uppermost surface of the sheets, number of which decreases as the sheets are fed, the sheet feeding device includes a detection unit 3 which comes into contact with the uppermost sheet and detects the height of the sheets. The detection unit 3 includes an arm portion 4 which is rotatable to bump against the uppermost sheet, and a sensor which detects the rotation of the arm portion 4, and detects the movement amount of the arm portion 4 rotating due to a decrease in the number of the sheets by a photo sensor or the like. Then, the distance h between the position of the uppermost surface of the sheets and the belt 2a is adjusted by moving up a bottom plate 6 using an elevation unit or the like on the basis of a signal from the detection unit 3.
Here, the sheets are evenly aligned at the front end thereof on the sheet feeding tray 21 in accordance with the size of the sheets. Further, as shown in FIG. 7, the attachment position of the arm portion 4 is set to the vicinity of the rear end of the sheet so as not to be easily influenced by the wind blowing from the blowing unit 1.
In addition, side fences 23 are provided next to both side ends of the sheets, and the side fences 23 guide the side ends of the sheets so that, when the uppermost sheet is fed, straight movement of the sheet is maintained. An end fence 10 is provided at the back of the rear ends of the sheets, and the end fence 10 prevents the sheets from being extruded backward by the blowing unit 1. The end fence 10 is configured to be movable in the forward/backward direction using a slide rail, a guide, or the like so as to be movable in accordance with the size of the stacked sheet. Further, the end fence 10 is fixed with the help of a gear or a screw.
A transportation roller 8 is disposed on the downstream of the suction belt unit 2, and transports the sheets arriving at the transportation roller 8. The transportation force of the transportation roller 8 is set to be larger than that of the suction belt unit 2. Further, a sheet feeding sensor 9 to detect the arrival of the sheet is provided on the downstream of the transportation roller 8.
The sheet feeding device 20 operates as follows. FIGS. 8 to 11 are cross-sectional views illustrating the sequence of the operation of the sheet feeding device. The blowing unit 1 is activated to blow air toward the end portions of the sheet when a sheet feeding command is sent from the image forming apparatus body, as shown in FIG. 8. Then, at the same time, the air suction of the suction belt unit 2 starts. Thus, the uppermost sheet P1 is lifted and the uppermost sheet P1 is sucked to the belt 2a of the suction belt unit 2 as shown in FIG. 8.
Subsequently, the driving of the suction belt unit 2 and the transportation roller 8 starts as shown in FIG. 9, and the driving of the suction belt unit 2 stops after the sheet P1 arrives at the sheet feeding sensor 9 as shown in FIG. 10. Then, the transportation roller 8 continues to transport the sheet P1 while the belt 2a of the suction belt unit 2 stops.
Subsequently, as shown in FIG. 11, the sheet P2 is lifted by air and sucked directly after the sheet P1 exits from the suction area. Then, in accordance with the feeding interval which has been set, the driving of the suction belt unit 2 is resumed, and the feeding of the sheet P2 is performed. By repeating the above-described operations, the sheets are sequentially transported.
Here, in the sheet feeding device that separates the sheets with air to transport the sheets, when separating the sheets that adhere to each other with high adhesion force, it is general that the wind speed of the air from the blowing unit 1 is increased or the nozzle opening of the blowing unit 1 is enlarged to increase the volume of the wind blowing to the sheets so that the adhesion of the sheets is eliminated. However, in the case of feeding thin and highly adhesive sheets such as thin coated sheets, if the volume of the wind is increased as described above, a volume of air, which is larger than necessary to lift the sheet, is sent between the sheets.
As a result, a flaw may arise in the above-described sheet feeding device of the related art. FIG. 12 is a plane view illustrating the flaw of the sheet feeding device of the related art, and FIG. 13 is a cross-sectional view illustrating the flaw of the sheet feeding device of the related art.
That is, in the sheet feeding device of the related art, since the end portions of the sheets are in an area that is almost closed by the sheet blocking member 7, the side fences 23, and the end fence 10; the excessive air blowing between the sheets is blocked by the side fences 23 and the end fence 10 as shown in FIG. 12 and all of the air cannot exit from the end portions of the sheets, so that the remaining air accumulates between the sheets. When the air accumulates between the sheets in this way, the sheets are lifted upward in a bundle due to the accumulating air as shown in FIG. 13. Then, the sheet bundle is strongly pressed to the suction belt unit 2. In this state, when the driving of the suction belt unit 2 starts to feed the sheets, the sheets are fed in a bundle, which causes a double feeding. This causes waste of sheets and/or an undesirable event in which white sheets are mixed with printed sheets. As a result, the quality of the image forming apparatus is prominently degraded.